1. Field of the Invention
The present invention relates generally to the field of optical fiber cables and more particularly to a spliceless optical fiber cable having at least one stub or drop cable extending therefrom.
2. Description of the Prior Art
With the advent of local area networks and the relative broadband capabilities of fiber optic links, it has become commonplace for new communication systems to include fiber optic capabilities. In the use of optical fibers, optical fiber cables are provided for physical protection of the fibers in view of the fragile nature of the glass optical fibers. A main distribution cable, which is normally deployed underground, is typically used between two distant termination points in a communication system. At a point where a connection must be made to local homes or buildings, a splice is made to one or more optical fibers in the distribution cable for connecting a separate smaller-sized cable for connection with the local termination point. The smallersized cable may be either a drop cable, which is directly routed from the distribution cable to the local home or building or a stub cable, which is routed from the distribution cable to an above-ground pedestal or an optical network unit (ONU) for subsequent connection to a plurality of local termination points by drop cables.
In those localities where a plurality of local termination points are to be connected into the system, it is more desirable to utilize a stub cable rather than having a plurality of drop cables spliced to various branch points along the distribution cable. A stub cable is an optical fiber cable, normally shorter in length than a drop cable, having typically two or more optical fibers therein. The stub cable is spliced at a branch point to the distribution cable. It is then routed to a pedestal, an ONU, or other optical equipment for subsequent connection to the local termination points via drop cables. A pedestal is used as an above-ground location for making splices to the various drop cables which are utilized in the immediate locality. The use of the pedestal to house the splices and to provide access to the distribution cable avoids the necessity and inconvenience of digging up the distribution cable if, for instance, a splice needs reparation or drop cables need to be added at the branch point as the local communication system expands. This saves time and expense and avoids inadvertent cable damage by earth-moving equipment. An above-ground ONU is utilized for similar reasons as the pedestal but has the additional capability of converting optical modulation to electrical modulation, and vice versa, if this is necessary. Besides providing above-ground access to the distribution cable, the use of stub cables is highly desirable so that the distribution cable has as few openings in its outer protective elements as possible for minimizing the risk of environmental damage such as water ingression.
In the past, however, in those systems in which stub or drop cables are required, typically, a splice to the distribution cable must be made on site at the specified branch point. Making an onsite splice sometimes requires that the technician making the splice overcome such adverse working conditions as extreme weather and inconvenient distribution cable placement. Further, making the on-site splice is a very difficult and time-consuming procedure which entails accessing the cable at the desired branch point, carefully opening the outer sheath, radial strength yarns, buffer tubes, etc., for accessing the specified optical fibers, splicing the specified optical fibers to the stub or drop cable optical fibers, and sealing the branch point. A splice closure, normally comprised of metal, is thereafter enclosed around the branch point for providing the splices with protection from mechanical damage, and for providing an adequately strong anchoring point for the stub or drop cables.
The conventional splice closure, which must house slack buffer tubes and optical fibers without violating the minimum bend radius of the optical fibers, is relatively large so that it may provide adequate space for storage and splicing purposes. Because of its size, the conventional splice closure is impractical for all other uses except for on-site optical fiber splicing.
For example, in those systems in which the cable route architecture has been determined, preterminated cables utilizing copper conductors or the like have been designed and utilized. A preterminated cable comprises a main distribution cable and several stub and drop cables spliced to the distribution cable at various specified branch points during the initial manufacture of the cable. The stub cables, which are normally between 10 and 20 feet in length, may then be connected to a corresponding pedestal or ONU upon cable deployment. Similarly, the drop cables, which are usually less than 100 feet in length, may be connected to specified termination points upon cable installation. This facilitates the installation of cable thereby minimizing the time and cost. The stub and drop cables may also be preconnectorized, or be assembled with the appropriate connectors at the time of manufacture, for easier and faster installation.
But, in the past, preterminated optical fiber cables have not been manufactured or utilized because of the simple reason that conventional optical fiber splice closures are large and bulky thereby making them impractical for preterminated use. Because the optical fiber distribution cable may be several thousand feet in length, it is normally wound in reels upon manufacture so that it may be stored for subsequent deployment. The conventional splice closure makes a preterminated optical fiber cable quite impractical and unwieldy to be installed in this manner. In addition, an optical fiber cable utilizing conventional splice closures may not be placed on small cable reels for deployment without incurring cable or fiber damage.
Furthermore, in many applications, it is desirable that the optical fiber cable be disposed within polyvinyl chloride (PVC) piping for additional environmental and rodent protection. Because standard PVC piping has a small inner diameter, it is impossible to install a preterminated optical fiber cable utilizing the conventional splice closure. In such a situation, the only manner of providing stub or drop cable access to the distribution cable using the conventional splice closure is by making the splice after the distribution cable is deployed. Thus, providing a preterminated optical fiber cable having stub or drop cables spliced thereto while maintaining optical fiber protection and minimal overall cable size is a difficult problem which has only been recently resolved by the preterminated fiber optic cable as disclosed in a co-pending, related patent application of Nilsson et al., entitled PRETERMINATED FIBER OPTIC CABLE, filed on even date herewith.
In Nilsson et al., a preterminated fiber optic cable comprising a main distribution cable having a plurality of drop cables spliced thereto is disclosed. The unique concept of providing storage for slack optical fiber disposed outside of the corresponding buffer tubes allows the splice closure of the Nilsson et al. preterminated optical fiber cable to retain a small outer diameter (less than 4") and short overall length (less than 7") thereby allowing the preterminated cable to be deployable in most instances.
However, there are circumstances where it is desirable to use smaller PVC piping through which even the Nilsson et al. preterminated fiber optic cable will not fit. For example, if a local neighborhood with a plurality of specified termination points is to be outfitted with optical fiber cabling, a distribution cable would be routed, normally underground, in the neighborhood's vicinity. Normally, a stub cable would be spliced to the distribution cable and routed to an above-ground pedestal or ONU. There, the stub cable would be spliced to corresponding drop cables for connection with the local homes or buildings. In such an application, at initial deployment, it might be preferable to route the distribution cable through PVC piping having an inner diameter of as little as 1.25 inches so that as little excavation as possible would be required. This is especially true when existing paved streets must be excavated for cable deployment.
But, splice closures, by their very nature, must have, at the very minimum, an outer diameter of greater than two inches so that they may provide slack optical fiber storage without violating the fiber's bend radius of one inch. Therefore, in the past, the only manner of providing stub cable access to distribution cable to be deployed through 1.25 inch piping was to make a splice after the distribution cable has been deployed. As discussed above, such a method has a number of disadvantages. The technician making the splice may be faced with such adverse working conditions as extreme weather or inconvenient cable placement. Also, the buried splice is inconvenient for future access for rerouting, repair, reconfiguration, etc. Excavation for the purpose of cable access risks inadvertent cable damage.
Although highly desirable for such applications because of naturally small branch point size, spliceless preterminated cables have not been used in the past, mainly because the complex manufacturing required made the use of such cables impractical if not impossible.
As can clearly be seen, providing a spliceless preterminated optical fiber cable is a difficult problem which has not been previously resolved and even with the current level of understanding of optical fiber cables, there has not previously been a practical spliceless preterminated optical fiber cable although such a cable is desirable.